A human invertebral disk maintains a linkage between adjacent vertebrae of the vertebral column. It must fulfil a number of important functions including load bearing and dampening of impact forces. Furthermore, it must permit a complex pattern of movements and resist various stresses, pure or combined, in the sagittal, coronal and axial planes. Assisted by musco-ligamentous structures surrounding the spine, the invertebral disk must also help to maintain the normal alignment of the vertebrae of the spinal column.
An ideal artificial disk replacement will accurately reproduce all the functions of the invertebral disk. However although there have been many different artificial disks which have been described and tested, at this time they have all failed to reproduce the abilities of an invertebral disk.
Typical failings of previous artificial disks have included loosening or dislodgement of vertebral fixation, premature materials wear or structural failure, poor replication of normal or physiological spinal segmental motion and predisposition to the loss of normal neutral vertebral alignment.
An important aspect of the normal motion of the spinal column and the kinematics of the various invertebral motion segments is the behaviour of the motion segments during flexion and extension movements in the sagittal plane. Fundamental to the kinematics is the location of the instantaneous axis of rotation (IAR). The IAR varies from level to level within the spinal column and throughout flexion and extension movements for any given motion segment (level).
One type of spinal disk prosthesis is described in U.S. Pat. No. 5,674,296. The endoprosthesis described consists of a resilient body having a generally elliptical shape. The endoprosthesis is affixed between adjacent upper and lower vertebrae through L-shaped supports each having confronting concave-convex legs for engaging the adjacent bone sectional thickness on one surface and retaining the resilient endoprosthesis therebetween. The endoprosthesis is centrally located between the upper and lower vertebrae to allow central pivoting of the upper vertebrae relative to the lower vertebrae.
In addition to the above a gasket and seal are located at the anterior and posterior regions between the vertebrae to seal the endoprosthesis in its position between the upper and lower vertebrae.
U.S. Pat. No. 5,556,431 describes another type of invertebral disk endoprosthesis in which top and bottom plates are used instead of the L-shaped supports of the above identified US patent. The endoprosthesis described includes a core which has spherical upper and lower surfaces which from drawings shown appear to be aligned with a central vertical axis through the upper and lower vertebrae.
In contrast to U.S. Pat. No. 5,674,296 the prosthesis core of this patent has an edge rim which limits the range of movement of the core and ensures even under extreme conditions cohesion of the prosthesis.
This patent also discloses displacement of the centre of articulation of the prosthesis towards the rear relative to the centre of the vertebral end plates so as to provide sufficient space in the ventral edge area of the prosthesis upper and lower plates so as to enable receipt of bone screws.
Other artificial prostheses have sought to reproduce normal variation in the location of the IAR using various mechanisms including the use of visco-elastic deformable cores. An example of this is shown in U.S. Pat. No. 5,824,094. Unfortunately these type of artificial disks are subject to premature materials wear and stress failure. Furthermore, artificial disks with metallic springs have not yet found their way into clinical use.
All of the artificial disks described above have inherent problems which ultimately create unnatural stresses and resultant pain for an artificial disk implant recipient. The present invention provides an alternative prosthesis which is aimed at mitigating at least some of the problems associated with prior art prosthesis.
The applicant's co-pending application identified by application number 2005901682 and entitled A Prosthesis is incorporated herein by way of reference.
A prosthesis is described in this patent application comprising an upper part for attachment to an upper vertebrae, a lower part for attachment to a lower vertebrae and a middle part located between the upper and lower parts. The centre of the radius of curvature of co-acting surfaces of all parts is offset rearwardly with respect to a central vertical axis through the upper and lower vertebrae.
The upper part which is in the form of an upper end plate typically has an upper surface which is connected to the lower surface of an upper vertebrae. Investigations have revealed that the upper end plate may suffer from subsidence. Typically plate failure occurs by tilting with the posterior aspect of the prosthesis subsiding. The anterior aspect has not been shown to subside. Furthermore, subsidence occurs almost exclusively at the upper posterior corner of the upper plate.